John's death left his heir aged nine and his kingdom in the throes of civil war. Much of the south-east was held by Louis, son of Philip of France, claiming the throne as husband of Blanche of Castile, granddaughter of Henry I. William Marshall was declared regent, Hubert de Burgh continued as Justiciar, and a number of the rebel barons transferred allegiance back to the young king. Two swift victories—one at Lincoln, the other a naval battle off Sandwich—persuaded Louis to abandon his claim. Henry reached his personal majority in 1223 at the age of sixteen, and declared himself independent in 1227. The new king was very different from his father. Though personally brave, he had neither taste nor ability for warfare. He was a generous patron to the church and devout in his personal religious life. His appearance was agreeable, his manner gentle, and he had a genuine interest in art and architecture. For all his political faults, he brought to the English monarchy a sense of dignity and a more heightened ceremonial.

In line with tradition, his first difficulties were with his own family. In July 1227, his eighteen-year-old brother, Richard, earl of Cornwall, seized a manor belonging to a royal servant and refused to yield it. He was supported by a number of barons and the king was obliged to give way. An early example had been set that the king might be defied. Two factors created unrest. There was much dislike among the English of the Poitevins whom Henry favoured and, after his marriage to Eleanor of Provence in 1236, to her Provenc,al relatives. Secondly, Henry had little success in warfare. He found Llywelyn ab Iorwerth, prince of Gwynedd, very difficult to deal with, and expeditions against him in 1228, 1231, and 1232 were near-disasters. Nor were his attempts to reconquer the lost territories in France any more successful: an expedition in 1228 had to be aborted, and that of 1230, led by the king himself, achieved little. The cost of these campaigns was heavy, Henry's way of life was lavish, and his great respect for the papacy meant that papal exactions were severe. A quarrel in 1232 with Hubert de Burgh deprived him of an experienced and steadfast supporter. Henry resumed the war against Louis IX of France in 1242, but was chased back into Bordeaux. From 1248 he entrusted Gascony to his new brother-in-law, Simon de Montfort, who had married his sister, Eleanor, in 1238.

The crisis of Henry's reign began with foreign policy. In 1255 the pope offered the kingdom of Sicily to be held as a papal brief, which Henry accepted on behalf of his second son, Edmund. Few of Henry's subjects saw any advantage to themselves in it, and the baronage refused to make any grant in support. At the Parliament of April 1258, many of the barons appeared in armour, demanding the exile of foreign favourites, and a committee to control the king; at the head of the baronial claims was de Montfort, a brave and resourceful soldier. Henry submitted but, like his father, appealed to the pope to absolve him from his pledges. In 1263 the issue was put for arbitration to Louis IX who, in the interests of royal solidarity, found totally in favour of the king. Recourse to arms was then inevitable. In May 1264, at Lewes, Henry's troops were routed by de Montfort's men, and both the king and his young son, Edward, captured. For a year Henry was a puppet king, taken around in de Montfort's entourage. He was restored to power in August 1265 when Edward escaped, defeating and killing de Montfort at Evesham. In 1269 Henry attended a magnificent ceremony at Westminster abbey, which had been completely rebuilt, during which the shrine of Edward ‘the Confessor’ was dedicated. His last years were spent in failing health, and he died in 1272. Although, in the end, Henry triumphed over his adversaries, the long-term implications of his reign were that there were limits within which monarchs must work, and the constitutional safeguards against the abuse of power imposed upon him and his father, John, were not forgotten.